Bad Shaft Taper Leads to Vibrations

2011 March 21
by Chris Brown

A poorly machined coupler taper on a shaft will definitely lead to vibration problems.

We are working through a few issues with a customer that has had persistent vibration problems on his 80′ vessel. Once the shafts were out and in the machine shop we noticed an obvious problem with one of the coupler tapers. As the shaft was rotating in the shaft straightener, you could see that the fine line where the taper blends into the shaft had a visual wobble. This is a sign of a taper that was cut off-center. A few measurements confirmed that the center hole used to hold the shaft centered in the lathe for machining was off 0.020″.
That means the pilot on the coupler was running out 0.020″ (which was confirmed by putting the coupler back on the shaft) causing the shaft to run-out the same. In our business 0.020″ is a huge number that is definitely one source of running gear vibration.
The remedy was relatively simple – put the shaft in the lathe and recut the taper. We had to use the steady rest in the lathe since the center hole was useless. By recutting the taper the coupler will fit a bit further down the shaft which, in effect, shortens the shaft length. Fortunately we had a little length to play with so the only other change involved moving the Spurs line cutter holding block.  I will put up a post on this topic later.
Once the taper was recut the coupler was lapped on (the same way the prop was lapped in my prior post) for a perfect fit.  The coupler had to be refaced but amazingly the pilot ran-out 0.001″ without any additional machining.  The coupler was manufactured true. 

A short video will help understand visually what we saw.  Please excuse our amateur video editing – this is work in process.

Stay tuned for more info on this job and a final sea-trial report.

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